Marvin wants to destroy the Earth since according to him, it obstructs his view of Venus. He calls his weapon of mass destruction the Uranium Pu-36 Explosive Space Modulator or more popularly known as the Illudium PU-36 on its subsequent appearances. Claiming that it has a massive capability of destroying the planet Earth, the said weapon ironically is just a size of a mere red dynamite. | Marvin holding the Uranium Pu-36 Explosive Space Modulator Image source: Wikipedia.org |
In accordance to this, it can be assumed that Martians like him have a very long lifespan since Marvin is said to have been trying to destroy the Earth for more than a thousand years already.